This talk will take us through the origin and initial diversifications of animal life.
One theme that will be very prominent throughout is that of Konservat Lagerstätten, or sites of exceptional fossil preservation. Whereas 99% of the fossil record consists of bones, shells, teeth and other hard parts, these localities preserve soft parts, such as muscle and tissue. We will see just how important this is in general for palaeontology, but also in the study of this particular topic of the origin of animals.
We left off the previous talk with oxygen levels at ~10% PAL. Another rash oxidation event happened at ~700 Ma which led to the atmosphere reaching a similar oxygen level to today. This is correlated with the break-up of the supercontinent Rodinia.
This break-up caused, or played into, several severe changes happening in the geosphere at the time. The Earth was coming off the grips of a Snowball Earth period. So advanced was the glaciation that it was initially thought that the Earth was frozen right up to the Equator; now it is thought to be more of a “slushball Earth”. With the increasing temperatures and melting glaciers, the sea levels rose.
Accompanying this is the presence of drastic geochemical anomalies; the diagram above shows carbon isotope anomalies, but the same patterns can be seen with other geochemical elements, including sulphur.
As of the last glaciation, we have the earliest fossil record of animals. A pertinent question is whether these anomalies were the cause of the origin of animals, or whether the origin of animals caused the geochemical anomalies, or to which degree they influenced each other.
Diagram Source: Marshall, C. R. 2006. Explaining the Cambrian “Explosion” of Animals. Annual Review of Earth and Planetary Sciences 34, 355-384.
As I mentioned, Konservat Lagertätten are important when discussing this topic, and here you can see why: there are a lot of them in this period of time. We will see why later. The ones in bold are the ones we will look at specifically.
Bitter Springs preserves cells in 3D, and I include it here as it is 800 Ma, but contains no animals.
Dengying contains the earliest true Ediacarans (see later).
Uratanna is the original Ediacaran Fauna formation, where they were first discovered.
It is thought that their sister group is the choanoflagellates, a group of flagellated unicellular eukaryotes. As the picture above shows, they can clump together to form multicellular colonies, and it is imagined that this is how the first animals (characterised by multicellularity) could have joined together from their unicellular precursors.
So now we can start looking at the first confirmed animals: the Ediacaran Fauna.
The diagram above shows their temporal distribution. The earliest fossils are small, from 635 Ma localities. These were interrupted by a last glacial period, after which the Ediacaran faunas radiated.
Diagram Source: Xiao,S. & Laflamme, M. 2009. On the eve of animal radiation: phylogeny, ecology and evolution of the Ediacara biota. Trends in Ecology & Evolution 24, 31-40.
Picture Source: Hofmann, H. J., Narbonne, G. M. & Aitken, J. D. 1990. <a href=”http://dx.doi.org/10.1130/0091-7613(1990)0182.3.CO;2″>Ediacaran remains from intertillite beds in northwestern Canada. Geology 18, 1199-1202.
Charniodiscus, Charnia and Rangea are referred to as problematics, i.e. we don’t know what they are. They bear a superficial resemblance to sea pens, but they have nothing to do with them systematically, or with any other taxon known. As such, we place them as separate phyla in their own right.
Tribrachidium is similarly problematic, however some authors place it among the medusoids (jellyfish).
The others are at the focal point of a lot of research, as they are thought to be stem-group representatives of the phyla that radiated in the Cambrian Radiation, i.e. of the animals besides sponges and cnidarians. Kimberella is thought to be a stem-group mollusc, Parvancorina a stem-group arthropod, Dickinsonia a stem-group mollusc or annelid, and Spriggina a stem-group annelid. These are all very tentative, though, and a full discussion would require a talk of its own.
In the past, a popular hypothesis pushed by Adolf Seilacher gained prominence: the Vendobiont hypothesis. Adolf Seilacher is a pioneer and revolutionary of trace-fossil palaeontology, and he postulated that the Ediacarans were something like colonies of unicellular organisms forming layers separated by air, like air-mattress organisms. This is now discounted even by Seilacher, as there is significant evidence that the Ediacaran faunas were animals, not just aggregated unicellular organisms.
Another hypothesis states that some of the fossils are actually algal or lichen trace fossils. This may sound outlandish, but experiments in algal and lichen taphonomy (grow them in similar mud and see what trace they leave behind) shows that their death trace bears an uncanny resemblance to some of the Ediacaran fossils, particularly Dickinsonia and the “compressed jellyfish”.
However, it is undeniable that animals were present amid the Ediacaran biota. The proof of this comes courtesy of the first of the very significant Konservat Lagerstätten, the Doushantuo Formation, China. Fossils from Doushantuo are all microfossils: nothing above a certain size gets preserved. However, these microfossils are impeccable, preserved in 3D and preserving cellular detail. Among them, we have found these microfossils pictures above.
These are embryos, which I sorted by cleavage stage. The only organisms that produce embryos like this are animals.
However, there is no extant animal taxon that has this cleavage pattern, so these embryos are not sortable systematically.
2012 update: many of these have been shown not to be embryos, but clustered single-cells organisms!
Pictures Source: Xiao, S., Zhang, Y. & Knoll, A. H. 1998. Three-dimensional preservation of algae and animal embryos in a Neoproterozoic phosphorite. Nature 391, 553-558.
From now we will look at the Cambrian Radiation of animals, starting with the three classic localities documenting it, before reviewing the causes.
The earliest of these localities is the Chengjiang outcrop of China. On the palaeogeographical map, it would be on the ocean floor on the outskirts of Gondwana, near where it says “South China”, whereas the Burgess Shale would be near Laurentia (where it says “Alaska”).
The fact that they are so wide apart, but are still so similar (as we will see) makes it reasonable to conclude that what is observed there are representative of the situations around the globe.
Map source: Scotese Palaeomap Project
Chengjiang and the Burgess Shale are very similar in every way, as seen in the diagram on the right with faunal compositions. 65% of the fossils there are arthropods – and almost all of them are non-mineralised, i.e. they are all soft parts.
Among them we can count all the main ecological guilds, from deposit feeders to carnivores.
Besides arthropods, we also find sponges, brachiopods and various “wormy” taxa. Also, the earliest chordate, Haikouella, is found in Chengjiang.
By far the most abundant Chengjiang fossils are these small, bivalved crustacean-like organisms called bradoriids. These can be imagined as swimming in swarms, like the krill of today, and probably served as the main food source for predators.
Picture Source: Hou, X., Williams, M., Siveter, D. J., Siveter, D. J., Aldridge, R. J. & Sansom, R. S. 2010. Soft-part anatomy of the Early Cambrian bivalved arthropods Kunyangella and Kunmingella: significance for the phylogenetic relationships of Bradoriida. Proceedings of the Royal Society B 277, 1835-1841.
The chronologically next Lagerstätte is the Sirius Passet of Northeast Greenland.
Sirius Passet is most well-known for its small shelly fauna. These include fragments of larger organisms, as well as whole organisms. As the name suggests though, they are all below a certain size and are essentially microscopic.
Kerygmachela is a stem-group arthropod.
Halkieria is thought to be a mollusc, but could also be an annelid. It has two shells (‘as’ = anterior shell, ‘ps’ = posterior shell), and the soft body is slug-like between them. The body is surrounded by sclerites (‘sc’).
One particularly common group of enigmatic microfossils are the mobergellans. They are little shells with 9 muscle scars on them; no such arrangement is known from other taxa, fossil or recent.
Some history on the place: it was discovered by Charles Doolittle Walcott, a Canadian geologist, in 1901. He was out looking for trilobites when he stumbled on peculiar fossils. He later got permission to dig out a quarry there and by 1916 had accumulated and described hundreds of specimens, and left thousands more in the archives of the Canadian Geological Department.
Through no fault of his own, the fossils were inadequately described – phylogenetic thought was not known or popular at the time. In the 1970s, Harry Whittington and his two students Derek Briggs and Simon Conway Morris started redescribing all these specimens in a brand new way: not as “weird animals”, but as stem-group representatives of modern phyla.
This is, of course, the correct approach (although some disagreed, notably Stephen J. Gould), and it was this reinvestigation, which is still ongoing with a new generation of students, that led to the discovery that there was indeed a Cambrian radiation of animals. Keep in mind that this was the first Cambrian Konservat Lagerstätte to be discovered, the rest came later.
One can imagine the Burgess Shale as being a somewhat deeper water environment, with all the organisms either in, on or swimming above the ocean floor. The habitat was right beneath a cliff. At some point, this cliff got dislodged, perhaps due to an earthquake, leading to an underwater mudslide which buried the entire habitat.
The main organism group we find there are arthropods, especially this one, Marrella.
Also among the significant organisms are the first cephalochordates (Pikaia) and early echinoderms (sea stars, brittlestars, sea urchins, sea cucumbers and sea lillies are the five classes still alive; there were 42 classes in their history, many of which had very strange morphologies, e.g. the helicoplacoid above).
Picture Source: Skovsted, C. B., Brock, G. A., Lindström, A., Peel, J. S., Paterson, J. R. & Fuller, M. K. 2007. Early Cambrian record of failed durophagy and shell repair in an epibenthic mollusc. Biology Letters 3, 314-317.
This is also obvious from the number of ecological guilds preserved, which span the entire possible spectrum.
The fossils of the Burgess Shale are very well-known for their weirdness [Note: yellow rocks above are Chengjiang ones]. It is a veritable freakshow. What will now follow is a simple showcase of the more famous fossils.
Anomalocaris is the poster child of the Cambrian Radiation, a large and probably vicious predator with two large eyes, an enormous toothy appendage and a characteristic pineapply mouth,
Originally, it was described as two animals, the appendage being one animal and the pineapple mouth another. It wasn’t until the Whittington et al. reinvestigations that it was discovered to be one single animal.
Anomalocaris is the original, but many more relatives have been found with very different morphologies. Only three have body fossils: Anomalocaris (top left), Hurdia (top right) and Laggania (bottom left). The rest are known from isolated appendages.
As a sidenote – going into the stem-group arthropod fossil record and systematics is impossible in this space – this appendage is called the great appendage, but it evolved in other taxa independently, although they are erroneously lumped together as “great appendage arthropods“.
Hallucigenia, as the name suggests, is a very strange animal. Originally, it was described upside down, but then was flipped over. It is a stem-group velvet worm (Onychophora) with spikes on its back, presumably for defence.
Opabinia is the classic freak of the Cambrian Radiation – when its reconstruction was first unveiled at a scientific conference, the auditorium reportedly burst out laughing. And it’s not hard to see why, with its five stalked eyes and that ridiculous proboscis ending in a grasping appendage.
To move away from the arthropods, we also have Wiwaxia. It’s an animal whose body is entirely covered by sclerites, and it also possesses those elongated spines. There is no fossil available yet to shows us what lay beneath all the armour. It is assumed to be either a stem-group mollusc or annelid.
And for a brand new reconstruction, here’s a reinterpretation of Nectocaris pteryx as a cephalopod.
It also has a varied history, first described an an anomalocarid, then as an arrow worm, before ending up as a cephalopod. This is a testament to the difficulty of getting the interpretations of these ancient fossils right.
So basically, when you see a picture like this, the only thing that is fictional are the colours. No matter how strange the organisms in a Cambrian reconstruction look like, they are most likely to be real.
It was the first appearance of bilaterian animals in the fossil record. The Bilateria is the node numbered 573 above – all animals excluding Cnidaria (jellyfish) and Porifera (sponges). Before the Cambrian, what was present was the stem-group of all these phyla. But it was only in the Cambrian that the crown groups started appearing.
Source: Barton, N. H., Briggs, D. E. G., Eisen, J. A., Goldstein, D. B. & Patel, N. H. 2007. Evolution. CSHL Press. [See http://evolution-textbook.org/]
The Cambrian Radiation is the result of a new innovation in developmental biology: triploblasty, i.e. a third germ layer. In sponges, there is only one germ layer, the endoderm, which builds the digestive system and epithelial linings (and lungs, in those animals who have those). Cnidarians go one step further and have an ectoderm, out of which arises the nervous system.
Bilaterians all have three germ layers with the addition of the mesoderm. This allows some very significant changes in morphology to happen, as the skeleton, the coelom and the circulatory system arise from it.
In abstract terms, what these lead to is an expansion of the possible morphospace. In practical terms, there is more support available for differing morphologies: with muscles come new ways of moving, circulatory systems allow larger body sizes, coeloms provide structural support.
There were two major changes in the nature of the fossil record in the Cambrian. The first is the appearance of massive biomineralisation – the building of shells and carapaces.
This not only plays into the support-for-morphologies aspect (and other aspects, as we will see), but is also a fundamental change for the fossil record, as it is the beginning of ‘regular’ fossilisation.
Obviously, biomineralisation started out small. There are isolated examples from the Ediacaran fauna, but mineralisation first became popular with the rise of the small shellies in the Tommotian stage of the Early Cambrian.
For some background, biomineralisation is not simply a process by which a lump of hard material is thrown on an organism’s back – although it may have started that way. It’s a tightly-regulated process. Even in snails and bivalves, the shelled molluscs, the shells do not consist solely of minerals, but the minerals are integrated into a complex cocktail of proteins. In arthropods such as shrimp, biomineralisation is also present in the head shield, where trace amounts of carbonates are embedded in the chitin matrix. Vertebrate bones are the same: they are not clumps of calcium phosphate, they are living tissue.
Anyway, the appearance of biomineralisation can easily be linked to the geological changes of the time. With the break-up of Rodinia, more nutrients were leeched into the oceans and they became saturated. Combined with the warming of the climate, crystallisation was favoured. This was incorporated and controlled by the organisms because biomineralisation is very useful for shells and for weapons, as we will see.
The other major change is in the trace fossil record. The picture shows Trichophycus pedum, a trace fossil whose earliest presence at 535.5 Ma is the accepted boundary between the Proterozoic and the Cambrian periods.
Note that it has now been found at earlier times, but the generalisations in the next page are still valid.
Trichophycus is a deep and very sculpted trace, something typical of a burrowing animal. Traces like these are only found from the Cambrian onwards – Ediacaran trace fossils are mere scratches on the surface.
Such an advance in a trace fossil can only be brought about by advances in morphology – this is the effect of triploblasty and the development of muscles and active movement.
In turn, this leads to what is referred to as the “Cambrian substrate revolution”. Animals could now invade a new, up to now unexplored territory: the sediment. They could anchor themselves or dig tunnels. In essence, the increase in trace fossil complexity heralds the start of regular marine ecology, and it can all be traced back to the development of triploblasty.
The deglaciations caused flooding and a great increase in the space available for animals to live in. At the start, these were colonised by the Ediacaran faunas, which formed what is called a permissive ecology, i.e. one in which even inefficient organisms like the Ediacarans could survive.
However, their reign was ended due to the rise of oxygen. Oxygen was the key to taking advantage of triploblasty: it allowed circulatory systems to develop and body size to increase, by allowing the biosynthesis of many types of tissues and fibers, most notably collagen. These, combined together, allowed the triploblasts to rise to dominance.
Another aspect to keep in mind is the evo-devo (evolution of development/evolutionary developmental biology) one. It is well-known that many, if not most, genes, especially developmental ones, are conserved across all animal taxa. The early evolution of animals proceeded largely by cooption of existing genetic pathways, and a few select duplications.
This, combined with the environmental chaos, led to a whole load of experimentation, producing such weird freaks of nature as the vetulicolians, pictured above.
With natural selection, the successful “body plans” (I use that term loosely for the sake of pedagogy!) were kept and development was “canalised”, i.e. focused along narrower and narrower paths. In abstract terms, the few fitness peaks on the morphospace were gradually being discovered and climbed.
This plays into a larger issue in palaeontology and evolutionary research, namely how genetic changes result in morphological ones. This is another issue which would require a specific lecture (series) to deal with properly, so it is summarised in red: you cannot simply correlate genetic change to morphological change.
This may seem like an irrelevant quip to make for this topic, but whether the Cambrian Radiation is a real event is actually a controversial subject. Molecular phylogeneticists employ a molecular clock in order to measure divergence dates. This molecular clock is not at all as foolproof as is made out to be and is faulty more often than it is correct (for reasons that would require their own talk to explain!).
Earlier molecular clock studies had outrageous dates for the origin of animals (cf. above example). Nowadays, they are closer to the fossil record, but still have an (expected) gap, mostly placing the origin of animals between 700 and 600 Ma.
This has led to talk of the Cambrian Radiation being merely an illusion, brought about by a gappy fossil record. This is complete poppycock: you cannot wish for a better fossil record than that in the Cambrian. It is the molecular clocks that are wonky, not the fossil record.
Very basically, the molecular clock finds out when genetic intermixing stopped – i.e. the point of speciation. However, because genetic changes are not directly measurable in morphology, this is impossible to see in the fossil record.
Note that I am oversimplifying the matter to the point of perhaps being false. If wanted, contact me for more detailed info on the difference between molecules and morphology, and molecular clocks and the fossil record.
So by now we have our diverse animals that can move around in whole new ways. The driver of the Cambrian Radiation was how the animals employed their new abilities both in dealing with their environment and among themselves.
The Cambrian substrate revolution is the main novelty in the former, but it should not be forgotten that active swimming and movement was also a novelty.
However, the real star of the show is the latter: how animals started influencing each other. I mentioned the preservation of organismal interactions in the Burgess Shale. These can all be led back to a single selective pressure: predation.
Biomineralisation was selected for because hard shells offer protection. In response, predators took advantage of the same effect to make weapons. In response to those, animals lower on the food chain reinforced their shells. And so on. This pattern is called a coevolutionary arms race and it is most evident in the Cambrian Radiation.
Predation explains why we have animals such as Opabinia with five eyes, or anomalocarids with huge eyes: they were predators and needed to see their prey. The main use of shells is for defence, and they needed to be in top shape at all times, as evidenced by this particular picture with the shell damaged by predation. Even the Cambrian substrate revolution played into the predation aspect: burrows are great hiding spots.
In a nutshell, environmental changes triggered the radiation, developmental biology and geochemistry allowed animals to diversify, and the radiation was made all the more spectacular by all-new types of ecological interactions, most importantly predation.
To move on from the Cambrian Radiation, we can look at the next striking Konservat Lagerstätte: the Orsten. Initially, it referred to a single locality from the Late Cambrian of Sweden, but it is now known as a special taphonomic window (i.e. a place that preserved specific things) from all over the world.
It preserves small animals in small nodules with extremely fine detail and in 3D, as seen above. The majority of these are arthropodan larvae.
For more info: Maas, A., Braun, A., Dong, X.-P., Donoghue, P. C. J., Müller, K. J., Olempska, E., Repetski, J. E., Siveter, D. J., Stein, M., Waloszek, D. 2006. The ‘Orsten’—More than a Cambrian Konservat-Lagerstätte yielding exceptional preservation. Palaeoworld 15, 266-282.
For pictures and updates: http://www.core-orsten-research.de/
As an example of how spectacular these fossils are, here’s Cambropycnogon, the first instar larva of a pycnogonid (sea spider).
Even more informative, we have larvae of stem-group crustaceans, which provides us with a lot of info and data on the systematics and evolutionary history of the crustaceans that is otherwise unobtainable.
So by the end of the Cambrian, this is what the sea-floor looked like: it was by and large arthropod-dominated, with trilobites being the most diverse group, some competing with the hyoliths (those conical animals with the handlebars). A couple of epibenthic bivalves were also common.
Picture source: Harper, D. A. T. 2006. The Ordovician biodiversification: Setting an agenda for marine life. Palaeo3 232, 148-166.
The Cambrian had a unique type of fauna, with low species diversity. With the beginning of the Ordovician, though, this changed as one of the largest radiations in the history of life on Earth took place.
It should be clarified that the Cambrian Radiation did not lead to such a huge increase in diversity, but was a diversification of “higher levels of systematics” (of phyla and form). However, it was in the Ordovician that the lower levels (orders, classes) known from the fossil record and today originated, as seen in the diagram above.
This radiation is explained by more environmental changes. In the Ordovician, the oceans rose more and huge epicontinental seas developed, leading to more colonisation space for the opportunistic organisms to take advantage of quickly.
Picture source: Zhan, R.-B., Jin, J. & Rong, J.-Y. 2006. β-diversity fluctuations in Early–Mid Ordovician brachiopod communities of South China. Geological Journal 41, 271-288.
Another group that radiated in the Ordovician were the trilobites, reaching high taxonomic, morphological and ecological diversity.
For more on echinoderm palaeodiversity: Smith, A. B. 2005. The pre-radial history of echinoderms. Geological Journal 40, 255-280.
What all these radiations led to was a drastic shift in the way marine benthic ecosystems are structured. The Cambrian seafloor was crawling with very mobile species; in the Ordovician, these were replaced by many mostly-sessile organisms such as sea lillies (crinoids) and bivalves. Brachiopods were at their peak, and molluscs were beginning to rise to dominance.
However, it is from this point that we start getting the effect of the Cambrian Radiation on the fossil record. The animals that radiated in the Ordovician all have hard parts and are preserved in any case.
Soft part preservation depends on anoxic, undisturbed deposition. With the advent of burrowing in the Cambrian Radiation, such depositional enbironments were hard to come by: any carcass that gets buried will just be fed on by an organism living in the sediment.
What this means is that this radiation of shelly- and skeletonised organisms, while it did definitely take place, may also have been accompanied by diversifications in non-shelly taxa, such as most arthropods (besides trilobites).
This highlights the importance of Konservat Lagerstätten, only one of which exists for the Ordovician: the Fezouata Formation in Morocco. What we find there, besides the shelly taxa, are remains of many soft-bodied arthropods, many of which can be linked back as relatives of the Cambrian freaks. For example, above you see Furca, a relative of Marrella, the most abundant arthropod of the Burgess Shale and most abundant arthropod of the Cambrian.
As a summary, here is what happened in the Ordovician Radiation. It is a classic case of shifting ecological dominance. The cause was the appearance of new and widespread shallow oceans, allowing the orthids (cephalopods, the straight-coned ancestors of the ammonites) and snails to radiate and push the previously dominant trilobite and early crinoid fauna into deeper waters. The same happened in the Late Ordovician, when bivalves took over and pushed those groups into mid-deep water.
However, this was not as drastic as it may seem. Sure, biodiversity took a big hit, but the new ecological structure that was established in the Ordovician Radiation re-emerged unscathed, just with other taxa filling those ecological guilds.
The cause of it is simple. These new oceans that had formed were exceptional, in that they were the result of melted glaciers. As soon as a cold climate came back into play, these oceans disappeared quite suddenly, and the shallow-water organisms had no time to move or adapt to the deeper waters to survive.
Here, organisms were buried in volcanic ash and preserved in 3D (like in Pompeii). However, there is one unique aspect: these organisms cannot be prepared out of their nodules. Instead, many very thin slices are made (on the several µm-scale), and all these slides are scanned and digitally stacked together to form a 3D model of the organism inside. An example of a slice can be seen on the left, middle.
Through the years, the technique has been improved, and we can now make such detailed pictures as the coloured ones above, showing 2 stem-group arthropods (left), a brachiopod with pedicle (middle) and a pycnogonid (right; modern analogue for comparison below).
Again, examination of the specimens in this locality provides data we would otherwise not have on diversity for many organisms, and in enough detail to make proper morphological analyses possible.
This is where Schinderhannes bartelsi comes in. It is an anomalocaridid from the Lower Devonian Hunsrück Slates, another Konservat Lagerstätte, and the only [at the time, see note] non-Cambrian anomalocaridid known. [Note added in press: just this week, an Ordovician anomalocaridid was published, from the Fezouata Formation, see above. Interestingly, I had predicted this during my talks, meaning that I have divine powers and thus you should send me money. Uhh, where was I? Right, reality…]
Without its finding, it would logically be assumed that the anomalocaridids died out at the end of the Cambrian. This is the true importance of the Konservat Lagerstätten.
And to round off the talk, some more about these Hunsrück Slates, since I’ve done some work in them myself (in the same team that described Schinderhannes, although I joined later than that). The locality is 405 Ma, from western Germany. The fossils come from three quarries, Simmern, Gemünden and Bundenbach; see the map above, prepared by Gabriele Kühl (Uni Bonn) based on Rothe (2005) and Mittmeyer (1980).
Mittmeyer HG. 1980. Zur Geologie des Hunsrückschiefers. In: Stürmer W, Schaarschmidt J & Mittmeyer HG (eds.). Versteinertes Leben im Röntgenlicht. Kleine Senckenberg-Reihe 11, 34-39.
As you can see, the quarry is now left alone and collectors can just walk in and pick up slabs of slate. However, be warned that the fossils are very, very rare, especially the spectacular ones. The picture above is by Alexandra Bergmann (Uni Bonn).
On the deposition, the Hunsrück Slates are the result of turbidites (underwater mud flows). They swept through from shallow waters to deeper waters, carrying organisms and habitats with them, before depositing them in an anoxic basin. The anoxia was caused by bacteria who fed on the carcasses. Importantly, these bacteria caused the muscles to be replaced by pyrite – the ocean waters were saturated with iron and sulphur. This is what causes the soft-past preservation: pyrite is a hardy mineral.
The Hunsrück Slate fossils are pyritised. Pyrite is a radiodense material, meaning X-Rays cannot pass through them. This means we can X-Ray them, and in fact, the first fossils to have been studied by X-Ray were Hunsrück Slate ones, just a couple of years after the discovery of X-Rays.
In recent years, the advent of computer tomography has made it possible to X-Ray them in 3D, as shown above. This can potentially provide more information, or make it easier to interpret enigmatic structures.
Basically, what I want to say is that palaeontologists are not hole diggers and stamp collectors, and we also need and use technology to get the most out of our materials.